Clarifying how to disable 'crash_log'

The current set of instructions mention 'crash_log' being left undefined
as a variable to disable it, but when that happens, lager starts it at a
default location.

This commit makes it obvious that the value must explicitly set to
'undefined' in order to disable crash_log.
Merge pull request #165 from ferd/master

Clarifying how to disable 'crash_log'